In an OFDM system, a high speed serial signal is divided into several parallel signals and are modulated using orthogonal sub-carriers for transmission and reception. Therefore, the orthogonal sub-carrier divided into narrow bandwidths undergoes a flat fading and accordingly has excellent characteristics for a frequency selective fading channel. Since a transmitting device maintains orthogonality between sub-carriers by using a simple method such as a guard interval interleaving, a receiving device does not need a complicated equalizer or a rake receiver generally used in a DS-CDMA (Direct Sequence-Code Division Multiplexing Access) method. The OFDM system with such advanced characteristics has been adopted as a standardized modulation type in a radio LAN, such as IEEE802.11a or HIPERLAN, and a fixed broadband wireless access, such as IEEE802.16. The OFDM system has once been investigated as one of applicable technologies of a modulation and demodulation/multiple access method in a UMTS (Universal Mobile Telecommunications system).
Recently, various multiple access methods based on the OFDM have been actively researched. The OFDMA system has been actively investigated and studied as a promising candidate technology for achieving a next generation mobile communication satisfying with user requirements remarkably enlarged such as an ultra high speed multimedia service. The OFDMA system uses a two dimensional access method by coupling a time division access technology to a frequency division access technology.
FIG. 1 illustrates an allocation of a radio resource according to the conventional art. Referring to FIG. 1, in a radio communications system, many users divide and use limited uplink/downlink radio resources. However, many users do not divide and use a radio resource that is allocated to one user. That is, there may not exist any method in which the same resource is allocated to two or more users.
For instance, in a TDMA (Time Division Multiplexing Access) system, a certain time interval is allocated to a user, and accordingly a scheduling is carried out such that only the user can use radio resources in the specific allocated time interval. In a CDMA (Code Division Multiplexing Access) system, the scheduling is also carried out so as to allocate a difference code for each user. In other words, one code is allocated to only one user. In the OFDM/OFDMA system, a certain user can use an allocated region that comprises a two dimensional map represented by time and frequency.
FIG. 2 illustrates a data frame configuration according to a conventional OFDM/OFDMA radio communications system. Referring to FIG. 2, a horizontal axis indicates time by a symbol unit, while the vertical axis indicates frequency by a subchannel unit. The subchannel refers to a bundle of a plurality of sub-carriers.
An OFDMA physical layer divides active sub-carriers into groups, and the active sub-carriers are transmitted to different receiving ends respectively by the group. Thus, the group of sub-carriers transmitted to one receiving end is referred to as the subchannel. The sub-carriers configuring each subchannel may be adjacent to one another or an equal interval away from one another.
In FIG. 2, slots allocated to each user are defined by a data region of a two dimensional space and refers to a set of successive subchannels allocated by a burst. One data region in the OFDMA is indicated as a rectangular shape which is determined by time coordinates and subchannel coordinates. This data region may be allocated to an uplink of a specific user or a base station can transmit the data region to a specific user over a downlink.
A downlink sub-frame is initiated by a preamble used for synchronization and equalization in a physical layer, and subsequently defines an overall frame structure by a downlink MAP (DL-MAP) message and an uplink MAP (UL-MAP) message both using a broadcasting type which define position and usage of a burst allocated to the downlink and the uplink.
The DL-MAP message defines a usage of a burst allocated with respect to a downlink interval in a burst mode physical layer. The UL-MAP message defines a usage of a burst allocated with respect to an uplink interval therein. An information element (IE) configuring the DL-MAP includes a DIUC (Downlink Interval Usage Code), a CID (Connection ID) and information of a burst location (for example, subchannel offset, symbol offset, the number of subchannels and the number of symbols). A downlink traffic interval of a user side is divided by the IE.
Alternatively, a usage of an IE configuring the UL-MAP message is defined by a UIUC (Uplink Interval Usage Code) for each CID, and a location of each interval is defined by a ‘duration’. Here, a usage by an interval is defined according to the UIUC value used in the UL-MAP, and each interval begins at a point as far as the ‘duration’ defined in the UL-MAP IE from a previous IE beginning point.
DCD (Downlink Channel Descriptor) message and UCD (Uplink Channel Descriptor) message refer to physical layer related parameters to be applied to each burst interval allocated to the downlink and the uplink, which include a modulation type, a FEC code type, and the like. In addition, Parameters required (e.g., K and R values of R-S code) are defined according to various downlink error correction code types. Such parameters are provided by a burst profile defined for each UIUC and DIUC within the UCD and the DCD.
On the other hand, a MIMO (Multi-input Multi-output) technique in the OFDM/OFDMA system is classified into a diversity method and a multiplexing method. The diversity method is a technique in which signals having undergone different rayleigh fading are coupled to one another by a plurality of transmitting/receiving antennas to compensate a channel deep between paths, thereby leading to an improvement of reception performance. A diversity benefit to be obtained by this technique is divided into a transmission diversity and a reception diversity depending on whether it is a transmitting end or a receiving end. When N-numbered transmitting antennas and M-numbered receiving antennas are provided, a maximum diversity benefits corresponds to MN by coupling MN-numbered individual fading channels in maximum.
The multiplexing method increases a transmission speed by making hypothetical subchannels between transmitting and receiving antennas and transmitting respectively different data through each transmitting antenna. Unlike the diversity method, the multiplexing method cannot achieve sufficient benefits when only one of transmitting and receiving ends uses a multi-antenna. In the multiplexing method, the number of individual transmission signals to be simultaneously transmitted indicates the multiplexing benefit, which is the same as a minimum value of the number of transmitting end antennas and the number of receiving end antennas.
There also exists a CSM (Collaborative Spatial Multiplexing) method as one of the multiplexing method. The CSM method allows two terminals to use the same uplink, thereby saving uplink radio resources.
Methods for allocating radio resources of the uplink or downlink in the OFDM/OFDMA system, namely, allocating data bursts are divided into a typical MAP method and an HARQ method according to whether the HARQ method is supported or not.
In the method for allocating the bursts in the general downlink MAP, there is shown a square composed of a time axis and a frequency axis. In this method, an initiation symbol offset, an initiation subchannel offset, the number of symbols used and the number of subchannels used are informed. A method for allocating the bursts in sequence to a symbol axis is used in the uplink, and accordingly, if the number of symbols used is informed, the uplink bursts can be allocated.
The HARQ MAP, unlike the general MAP, uses a method for allocating the uplink and the downlink in sequence to a subcarrier axis. In the HARQ MAP, only the length of burst is informed. By this method, the bursts are allocated in sequence. An initiation position of the burst refers to a position where the previous burst ends, and the burst takes up radio resources as much as the length allocated from the initiation position.
The OFDM/OFDMA system supports the HARQ using the HARQ MAP. In the HARQ MAP, a position of the HARQ MAP is informed by an HARQ MAP pointer IE included in the DL-MAP. Accordingly, the bursts are allocated in sequence to the subchannel axis of the downlink. The initiation position of the burst refers to the position where the previous burst ends and the burst takes up radio resources as much as the length allocated from the initiation position. This is also applied to the uplink.
FIG. 3 illustrates an uplink radio resource (data burst) that is allocated to a terminal using a typical DL-MAP according to a conventional art.
In case of a typical DL-MAP, a first burst subsequent to a position of the UL-MAP is allocated to the terminal. The UL-MAP allocates an uplink data burst by the UL-MAP IE.
In the CSM method of the OFDMA technique based on IEEE802.16d and e, a base station in the typical DL-MAP method informs each terminal of data burst positions by a MIMO UL basic IE with the data format as shown in Table 1, and allocates the same radio resource to each terminal.
In order to notice the use of the MIMO UL basic IE, UIUC=15 is used as an extended UIUC. There are 16 different values to be represented as the extended UIUC.
TABLE 1SizeSyntax(bits)NotesMIMO_UL_Basic_IE( ){ Extended DIUC4MIMO = 0x02 Length4Length of the message inbytes(variable) Num_Assign4Number of burst assignment For(j=0; j<Num_assign;j++){  CID16SS basic CID  UIUC4  MIMO_Control1For dual transmission capableMSS 0: STTD 1: SMFor Collaborative SM capableMSS 0: pilot pattern A 1: pilot pattern B  Duration10In OFDMA slots }}
The MIMO UL basic IE which is used for allocating the same uplink resource to two terminals is used for other conventional MIMOs. When a terminal has more than two antennas, the MIMO UL basic IE informs the terminals which method, namely, a STTD method for obtaining a diversity benefit or an SM method for increasing transmission speed, is used.
The CSM method in the OFDMA technique based on IEEE 802.16d, e can be embodied by the HARQ MAP for an HARQ embodiment. FIG. 4 illustrates an uplink radio resource (data burst) that is allocated to a terminal by using the HARQ-MAP according to a conventional art.
Unlike the method for informing every bursts by the DL-MAP, in the method as shown in FIG. 4, an HARQ existence is informed by an HARQ MAP pointer IE of the DL-MAP IE for an HARQ exclusive. The HARQ MAP pointer IE informs of a modulation of the HARQ MAP, and coding state and size of the HARQ MAP.
The HARQ MAP is composed of a compact DL-MAP/UL-MAP informing of position and size of the HARQ burst, and, in particular, uses a MIMO compact UL IE for the MIMO. The MIMO compact UL IE is used by being attached to a position subsequent to a ‘compact UL-MAP IE for normal subchannel’ for allocating the conventional subchannel and a ‘compact UL-MAP IE for band AMC’ for allocating the band AMC. As shown in FIG. 4, the MIMO compact UL IE has only a function of a previously allocated subchannel.
In the aforementioned conventional art, when additional radio resource is required by the increased demand of the uplink, there is no appropriate way to satisfy such requirement. In that case, adding a frequency resource may be considered. However, because a base station position must be considered and it affects on the entire system, it is not regarded as a preferred alternative for increasing uplink resources. More preferred method is to allow more than two user to simultaneously use the existing resources that are previously allocated to one user.